Hydroxyethyl cellulose (HEC) and other hydrophilic polymers, such as polyvinyl alcohol and vinylpyrrolidone copolymers, have been used in the paint industry as protective colloids to improve the mechanical stability of the composition. HEC is commonly used in emulsion polymerization involving vinyl acetate compositions. Recent publications reported that vinyl acetate monomer does not readily graft to HEC under normal emulsion polymerization conditions due to lack of reactivity of vinyl acetate monomer towards carbon radicals. (See U.S. Pat. No. 4,684,704; Craig, D. in Polymeric Materials Science and Engineering, 51, 172 (1984); 54, 370 (1986), etc.). The latexes prepared by emulsion polymerization of vinyl acetate or other vinyl esters and HEC lack mechanical stability.
Acrylic monomers are more reactive towards HEC and have a higher HEC grafting potential. A highly HEC grafted all acrylic emulsion latex would provide mechanical stability, excellent dry film performance, and good compatibility with many hydrophilic components in paint formulations. However, one of the issues of grafting HEC to acrylic monomers is the gelation problem during the grafting reaction in the emulsion polymerization. Although there are published references that teach the methods of grafting HEC onto acrylic polymers, most of those published references have some practical issues for commercial practice, because of poor reproducibility and stability, or low level of grafting, or odor issues associated with the use of waterborne regulators (chain transfer agents), such as 2-mercaptoethanols, in the polymerization process.
Various attempts have been carried out to produce HEC grafted acrylic polymer. For example, different additives have been added in the process. Craig, D. H. (“The Synthesis of Hydroxyethyl Cellulose-Stabilized Acrylic Latexes”, Polymeric Materials Science and Engineering, 51, 172 (1984)) teaches a method of making HEC grafted acrylic polymer latex using waterborne regulators, such as 2-mercapto ethanol. However, the use of waterborne chain transfer agents often results in an odor problem during and after the production. U.S. Pat. No. 3,876,596 to Grubert et al. teaches an emulsion copolymerization process of carboxylic acid esters and protective colloids in the presence of a relatively large amount of emulsifier mixtures. Too much emulsifier can have a negative effect on water-sensitivity of the system and causes foaming U.S. Pat. No. 4,670,505 to Craig and U.S. Pat. No. 4,659,771 to Craig disclose HEC-grafted polyacrylate dispersions produced by emulsion polymerization in the presence of water-soluble amino alcohol compounds, or substantially completely water-soluble monomers having conjugated unsaturation, respectively. However, Craig reports that the grafting reaction between HEC and polyacrylate polymer is reduced by these additives.
Other examples include a method of post treatment of acrylic emulsions with HEC and initiators disclosed in U.S. Pat. No. 6,596,804 to Edwards et al. U.S. Pat. No. 5,795,928 to Janssen et al. prepares an acrylic copolymer in the presence of a water soluble protective colloid with a weight average molecular weight of less than about 75,000. U.S. Pat. No. 4,845,175 to Lo uses hydrophobically modified hydroxyethyl cellulose. However, these acrylic emulsions do not produce paint formulations that have great mechanical stability and exhibit the resistance to color float and syneresis.
HEC grafted acrylic polymers taught in the art, however, have significant drawbacks. HEC is a hydrophilic and water soluble polymer. When HEC is grafted to acrylic particles, a dried film formed from HEC grafted acrylic latex often suffers from inferior water sensitivity. When the grafted HEC becomes a part of the dried film structure in the continuous phase, the water sensitivity becomes more apparent.
Hence, there remains a need for developing novel formulations and processes to produce a highly HEC grafted acrylic polymer dispersions with high polymer solids and desirable particle size distribution without gelling problems. There also remains a need to improve the water sensitivity of the HEC grafted acrylic dried film.